Boron nitride (BN) is a chemically inert non-oxide ceramic material which has a multiplicity of uses based upon its electrical insulating property, corrosion resistance, high thermal conductivity and lubricity. A preferred use is as a filler material additive to a polymeric compound, for forming a low viscosity encapsulating material, or as a low viscosity thermosetting adhesive for use in semiconductor manufacture or in formulating a cosmetic material. As presently manufactured, boron nitride is formed by a high temperature reaction of between inorganic raw materials into a white powder composition of BN particles having an hexagonal structure similar to graphite in a platelet morphology. The platelet morphology is for many applications undesirable and of limited utility. A conventional powder composition of BN particles has the physical attributes of flour in terms of its inability to flow. Accordingly, when added as a filler to a polymeric compound, the viscosity of the blended material increases significantly in proportion to the loading concentration of the BN additive. In some cases, at concentrations above 30% BN, the blended material can no longer be adequately dispensed from a mechanical dispenser such as a syringe.
JP Patent Publication No. 08-052713 discloses spherical bodies having a size of 10 mm or less for use as ball mills, formed from slurry comprising ceramic powder such as BN, polymerizable monomers such as polyvinyl, alumina powder, yttria powder, and a dispersant. JP Patent Publication No. 08-127793 provides a BN slurry having a low viscosity and improved adhesion formed by dispersing BN in an aqueous solution of a water-soluble nonionic cellulose ether and a polycarboxylic acid salt as a dispersant. JP Publication No. 06-219714 discloses a slurry formed by dispersing BN in a polyoxythylene-based nonionic surfactant.
U.S. Pat. No. 6,652,822 discloses spherical BN formed from precursor particles of BN suspended in an aerosol gas, which is directed to a microwave plasma torch. U.S. Pat. No. 3,617,358 discloses spheroid particles formed by flame spraying a slip or slurry of fine particles such as metal powder, ceramic powder, and the like, in a slurry comprising a binder for binding the flame spray particles, using a thermal spray gun and with acetylene as the combustible and carrier gas. The spheroid particles formed by the flame spraying technique of the prior art have a crush resistance of at least 0.7 grams. In the flame spraying process as used in the prior art, flame temperatures may range from over 3000° C. to 5000° C., which would cause ceramic materials such as BN to lose their properties as heated above the melting points.
Unfired BN tends to have poor thermal conductivity in thermally conductive applications. Thus, it is known in the art to sinter BN powder to high temperatures of at least 1900-2000° C. to enhance the properties of the final BN product, i.e., improving the purity of the BN and leading to crystal or platelet growth. In thermally loaded polymer applications, the thermal conductivity improves with BN platelet sizes.
In the present invention, the surface morphology and shape of conventional platelet BN particles are modified to form boron nitride agglomerated particles, bound by an organic binder having a rheology when spray dried. In one embodiment, such boron nitride agglomerated particles when filled into a polymeric compound at loading levels between 30 to 50 wt. % BN, the viscosity of the filled composition remains below 300 cp and preferably below a viscosity of 250 cp. In a second embodiment, the boron nitride agglomerated particles when sintered, form aluminum oxide coating that further enhances the thermal conductivity of the BN in thermally conductive applications.